The draft Paris text mentions a global temperature increase of 2 degrees C about 20 times. Sometimes followed by "or 1.5 degrees C", obviously an alternative. The usage is that these metrics are a "global temperature goal" (page 6). This is more obvious when set out in full: "holding the increase in global average temperature below 2 degrees C or 1.5 degrees C above pre-industrial levels" (page 5).

So we can understand 1.5 degrees C and 2 degrees C as the quantitative measures of the undefined phrase 'dangerous anthropogenic interference with the climate system' from the UNFCCC.

So the 2 degrees C limit can be described as the de facto goal of climate policy, or possibly the central pillar of climate policy a bit like the myth of the World Turtle on which the world of climate policy rests. But more about turtles later.

The alternative 1.5 degrees C limit was proposed in July 2009 before the 2009 Copenhagen Summit by AOSIS, the coalition of 42 of the world’s most vulnerable small island states as they considered a 2 degrees C limit would exceed a safe threshhold for their survival and protection.

Consequently the 2012 Doha UNFCCC meeting set up a Structured Expert Dialogue. This involved a series of meetings or dialogues between UNFCCC diplomats and selected Intergovernmental Panel on Climate Change (IPCC) authors. We will return to this 1.5 degrees C dialogue after a look at scientist's criticisms of the 2 degree C limit.

In 2011, the Tyndall Center's Kevin Anderson gave a presentation called 'Going Beyond Dangerous Climate Change: Exploring the void between rhetoric and reality in reducing carbon emissions' (You-tube, audio, slides).

Anderson said the science behind the 2 degrees C limit was now a decade out of date and that 1 degrees C was the new 2 degrees C in terms of a boundary with 'dangerous' impacts. Anderson criticised his fellow climate scientists for giving a 'rose-tinted' framing of emissions pathways that were compatible with continued growth of GDP and the 2 degrees C limit.

More disturbingly, Anderson said the 2 degrees C limit didn't really matter any more as all realistic emissions pathways were now pointing to a 4 degrees C rise in average global temperature by 2100. See also this summary by David Roberts of Grist.

If a 4 degrees C rise in average global temperature was a taboo subject for a while Anderson shattered that idea by organising a 4 degrees C conference and a '4 degrees and beyond' edition of the Royal Society's journal Philosophical Transactions.

Anderson said that these unproven geo-engineering technologies amounted to a 'Harry Potter magic wand' and that therefore most climate scientists were soft-selling the real difficulty of reducing greenhouse gas emissions sufficiently to avoid 2 degrees C of warming.

As we know, in 2013 and 2014 the three working reports of the Fifth Assessment Report of the International Panel on Climate Change (IPCC)) were published. These reports included a new set of global emissions reduction pathways, the representative concentration pathways (RCPs). There were only four pathways ranging from a (more or less) 'business as usual' scenario (4 to 6 degrees C by 2100) and down to the 'crash mitigation' pathway, Representative Concentration Pathway 2.6. That pathway is the only one consistent with the 2 degrees C limit.

The defined pathways have been replicated (repeatedly simulated) by different climate modelling groups on different climate models in order to provide likely outcomes (warming) expressed as probabilities. You know, median warming, warming between 90% confidence intervals or warming of no more than 2 degrees C above pre-industrial temperatures with a probability of 67%. It is these modelling exercises that are behind the IPCC's 1000 billion tonne carbon budget consistent with a 66% probability of staying within 2 degrees C of warming.

In the median RCP 2.6 scenario, GHG emissions from the world's energy system must become negative after 2070. Meaning that the world's energy system, currently some 35% of annual emissions, must operate so that it sequesters more carbon dioxide than it emits.

A recent paper in the journal 'Nature' Betting on negative emissions, concluded that the IPCC was betting on unproven technology in relying so much on bio-energy with carbon capture and storage. At this point I must say I am starting to worry about the turtles supporting the pillar of climate change policy. One of the authors of that paper Glen Peters argues that only in computer models will negative emissions limit global warming to two degrees C.

So where does this depressing news leave the UNFCCC's review of the 2 degrees C limit? The Structured Expert Dialogue wrapped up only last month on 11 May 2015 in a 182 page report. Which I wouldn't download. A ten-page summary by Climate Analytics tells us that the expert dialogue report agrees with island states and the less developed countries that that 2 degree C is too high and that the 1.5 degree C goal would reduce risks.

A final pertinent point is made by one of the SED IPCC authors Petra Tschakert that

"a single-index of climate change risk inadequately capture(s)) the complexity of the climate system, it also poorly reflects locally experienced temperature increases and extremes and hence the large variation across regions and continents. No single person or any single species faces a global average."

Roll on the BECCS. Roll on some more turtles. If negative emissions and BECCS represent emissions reductions policy resting on a supportive mythical turtle then the claim of feasibility for holding warming to 1.5 degrees C is Turtles all the way down.